1. Field of the Invention
The present invention relates to an area light source device that illuminates a liquid crystal display panel and to a liquid crystal display apparatus including the area light source device.
2. Description of the Related Art
Recently, in an increasing number of liquid crystal display apparatuses that selectively pass emission light from area light source devices and display images, a reflective sheet is disposed on the back side of a light guide in order to improve the efficiency of use of emission light from the area light source device and to meet a demand for higher luminance. The reflective sheet is, for instance, a film whose basic material is polyethylene terephthalate (PET). As a result, bending or waviness tends to occur on the reflective sheet.
On the other hand, a light guide in an area light source device is formed of an acrylic resin, a polycarbonate resin, etc. in various shapes in accordance with product specifications in consideration of the function and the intended application of the light guide. Since the light guide is formed of the resin material, there is a certain degree of warp or waviness due to heat, or warp or waviness due to forming.
In the case where the above-described reflective sheet is directly disposed on the back surface of the light guide, the following problem may arise. That is, due to bending or waviness of the reflective sheet and due to the effect of warp or waviness of the light guide, there may be a part (contact part) where the reflective sheet contacts the back surface of the light guide and a part (non-contact part) where the reflective sheet is not in contact with the back surface of the light guide. Between the contact part and non-contact part, the angle of reflection varies when light leaking from the back surface of the light guide is reflected toward the light guide. If each of the contact part and non-contact part has a relatively large area, non-uniformity in luminance may be caused on the light emission surface of the light guide. In the liquid crystal display apparatus including such an area light source device, non-uniformity in luminance may be visually recognized and the display quality may deteriorate.
In order to solve this problem, there has been proposed a method in which a projection-and-recess-shaped portion is provided on the back surface of the light guide, or projections are provided on the surface of the reflective member that is disposed on the back side of the light guide. Thereby, the area of the contact part is reduced, and non-uniformity in luminance is made difficult to perceive (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2004-12591).
However, if a reflective sheet having a mirror-finished reflective surface on its side that is opposed to the back surface of the light guide is adopted in order to further improve the efficiency of use of emission light from the light source, the relative position between the light guide and the reflective sheet may be displaced. Consequently, the reflective surface may contact and damage the projection-and-recess-shaped portion on the back surface of the light guide. In addition, if foreign matter enters between the light guide and the reflective sheet, both the reflective surface and light guide tend to be easily damaged by the foreign matter due to a displacement in relative position between the light guide and reflective sheet. As a result, light leaking from the light guide to the reflective sheet side cannot efficiently be used, and enhancement in luminance may be hindered.